Phil Mickelson said while he always believed in ending his title drought there was "a lot of doubt" following the veteran's history-making US PGA Championship success.

Mickelson became the oldest major winner in golf history thanks to Sunday's two-stroke victory at Kiawah Island in South Carolina.

The 50-year-old, who eclipsed Julius Boros (48 years and four months at the 1968 PGA Championship), had not won on the PGA Tour since 2019, while his last major triumph came via the 2013 Open Championship.

But Mickelson defied form and age to capture his sixth major crown and 45th PGA Tour trophy at the expense of Louis Oosthuizen and Brooks Koepka.

"It's been an incredible day, and I've not let myself kind of think about the results until now, now that it's over," Mickelson said during his news conference after posting a final-round 73 to finish six under.

"I've tried to stay more in the present and at the shot at hand and not jump ahead and race. I've tried to shut my mind to a lot of stuff going around. I wasn't watching TV. I wasn't getting on my phone. I was just trying to quiet things down because I'll get my thoughts racing and I really just tried to stay calm.

"I believed for a long time that I could play at this level again. I didn't see why I couldn't, but I wasn't executing the way I believed I could, and with the help of a lot of people, my wife especially, Andrew Getson and my brother Tim and Steve Loy, I've been able to make progress and have this week.

"It's very exciting because I've had a few breakthroughs on being able to stay more present, be able to stay more focused, and physically, I'm striking it and playing as well as I ever have but I haven't been able to see that clear picture.

"Although I believed it, until I actually did it, there was a lot of doubt, I'm sure."

Mickelson, who added to his 2005 PGA Championship title and broke through for his first Tour victory since the AT&T Pebble Beach Pro-Am in February 2019, said: "It's very possible that this is the last tournament I ever win. Like if I'm being realistic.

"But it's also very possible that I may have had a little bit of a breakthrough in some of my focus and maybe I go on a little bit of a run, I don't know.

"But the point is that there's no reason why I or anybody else can't do it at a later age. It just takes a little bit more work."

"Certainly one of the moments I'll cherish my entire life," he continued. "I don't know how to describe the feeling of excitement and fulfillment and accomplishment to do something when -- you know, of this magnitude when very few people thought that I could."